The present invention concerns a gearbox. The invention concerns also a vehicle that comprises the gearbox, a method for controlling the gearbox, a computer program to enable a computer to perform the method to control the gearbox, and a computer program product comprising program code of the program stored on a computer-readable medium.
Hybrid vehicles can be driven by a primary engine, which may be a combustion engine, and a secondary engine, which may be an electrical machine. The electrical machine is equipped with at least one energy store, such as an electrochemical energy store, for the storage of electrical energy, and regulating equipment in order to regulate the flow of electrical energy between the energy store and the electrical machine. The electrical machine can in this way alternate between working as an engine and as a generator, depending on the operating condition of the vehicle. When the vehicle is braked, the electrical machine generates electrical energy, which is stored in the energy store. This is generally known as “regenerative braking”, and it leads to the vehicle being braked with the aid of the electrical machine and the combustion engine. The electrical energy that is stored is later used for the operation of the vehicle.
An epicyclic gear normally comprises three components that are arranged in a manner that allows rotation relative to each other. These components are a sun gear, a planet wheel carrier and a ring gear. Knowledge of the numbers of teeth on the sun gear and ring gear allows the mutual rates of revolution of the three components to be determined during operation. One of the components of the epicyclic gear may be connected to an output shaft of a combustion engine. Thus, this component of the epicyclic gear rotates with a rate of revolution that corresponds to the rate of revolution of the output shaft of the combustion engine. A second component of the epicyclic gear may be connected to an input shaft to a gearbox. Thus, this component of the epicyclic gear rotates with the same rate of revolution as the input shaft to the gearbox. A third component of the epicyclic gear is connected to a rotor of an electrical machine, in order to achieve hybrid operation. Thus, this component of the epicyclic gear rotates with the same rate of revolution as the rotor of the electrical machine, if they are directly connected to each other. Alternatively, the electrical machine may be connected to the third component of the epicyclic gear through a transmission that has a gear exchange. In this case, the electrical machine and the third component of the epicyclic gear may rotate with different rates of revolution. At least one of the rate of revolution and the torque developed by electrical machines may be regulated in stepless increments. During operation, when the input shaft to the gearbox is to be given at least one of a desired rate of revolution and torque, a control unit calculates, given knowledge of the rate of revolution of the combustion engine, the rate of revolution with which the third component must be driven in order for the input shaft to the gearbox to be given the desired rate of revolution. A control unit activates the electrical machine such that it gives the calculated rate of revolution to the third component, and thus gives the desired rate of revolution to the input shaft to the gearbox.
By connecting together the output shaft of the combustion engine, the rotor of the electrical machine and the input shaft to the gearbox using an epicyclic gear, the conventional clutch mechanism can be avoided. During acceleration of the vehicle, increased torque is to be supplied from the combustion engine and the electrical machine to the gearbox and onwards to the driving wheels of the vehicle. Since both the combustion engine and the electrical machine are connected to the epicyclic gear, the greatest possible torque that can be supplied by the combustion engine and electrical machine will be limited by any one of these drive units, the greatest torque of which is lower than the greatest torque of the second drive unit, having taken the gear exchange between them into consideration. When the greatest torque of the electrical machine is lower than the greatest torque of the combustion engine, having taken the gear exchange between them into account, the electrical machine will not be able to produce a sufficiently large reactive torque to the epicyclic gear. Consequently, the combustion engine will not be able to transfer its highest torque to the gearbox and onwards to the driving wheels of the vehicle. The highest torque that can be transferred to the gearbox is in this way limited by the power of the electrical machine. This is made clear also by the equation known as the “planetary equation”.
There are disadvantages associated with using a conventional clutch that disconnects the input shaft to the gearbox from the combustion engine while gear-change processes are taking place in the gearbox, such as the heating of the lamellae of the clutch, which results in wear to the clutch lamellae and to increased fuel consumption. Furthermore, a conventional clutch mechanism is relatively heavy and expensive. Also, it occupies a relatively large space in the vehicle.
The document EP-B1-1126987 discloses a gearbox with double epicyclic gears. The sun gear of each epicyclic gear is connected to an electrical machine, and the ring gears of the epicyclic gears are connected to each other. The planet gear carriers of each epicyclic gear are connected to a number of gear pairs, in such a manner that an infinite number of gear steps is obtained. Another document, EP-B1-1280677, discloses also how the epicyclic gears can be bridged by a gear step arranged at the output shaft of the combustion engine.
The document US-A1-20050227803 discloses a vehicle transmission with two electrical machines, each one of which is connected to a sun gear in one of two epicyclic gears. The epicyclic gears have a common planet gear carrier, which is connected to the input shaft of the transmission.
The document WO2008/046185-A1 reveals a hybrid transmission with two epicyclic gears, whereby an electrical machine is connected to one of the epicyclic gears and a double clutch interacts with the second epicyclic gear. The two epicyclic gears interact also with each other through a cogged wheel transmission.